Molecules

Perovskite

This subject is previously unknown to this blogger who has difficulty even in pronouncing "Perovskite". The following information is gathered from ChatGPT in the learning process (in Italic; with added links, illustrations and occasional comments by yours truly). ChatGPT can provide reliable information from its vast database, if the question is sensible. It is simply "garbage in, garbage out".

Perovskite () refers to a class of materials that share a specific crystal structure originally found in the mineral perovskite, which is

composed of calcium titanium oxide (CaTiO3). This structure is known as the perovskite structure, characterized by the general formula ABX3, where 'A' and 'B' are cations of different sizes, and 'X' is an anion; X = O for Oxide Perovskites. (see periodic table. For example, Ca has 2s electrons in its outer sub-shell to donate, Ti has 2d+2s; while O needs 2 more electrons in p to complete the sub-shell, thus it is written as A2+B4+O2-3 = CaTiO3 for a stable molecule). It occur, for examples, in carbonate skarns, altered blocks of limestone, and accessory mineral in alkaline and mafic igneous rocks, nepheline syenite, melilitite, kimberlites and rare carbonatites. Perovskite is a common mineral in the Ca-Al-rich inclusions found in some chondritic meteorites.

Figure 12-37a Perovskite Mineral [view large image]

The very special properties of perovskites originates from the high mobility of the electrons within such material. The key factors include:

Crystal Structure:
Perovskites typically have a crystal structure of the form ABX₃. This structure is highly symmetric and allows for efficient charge transport. The structure minimizes the scattering of charge carriers (electrons and holes), allowing them to move more freely through the material.

Defect Tolerance:
Perovskites have a high tolerance to defects and imperfections in the crystal lattice. Defects in many materials can trap electrons and holes, reducing their mobility. However, in perovskites, the presence of defects does not significantly impede the movement of charge carriers.

High Dielectric Constant:
Perovskites often have a high dielectric constant, which helps to screen the Coulomb interaction between electrons and holes. This screening effect reduces the binding energy of electron-hole pairs, making it easier for them to move independently as free charge carriers.

Long Carrier Diffusion Lengths:
Perovskites exhibit long carrier diffusion lengths, meaning that electrons and holes can travel long distances without recombining. This is crucial for efficient charge transport and collection in solar cells and other electronic devices.

Low Effective Mass of Charge Carriers:
The effective mass of electrons and holes in perovskites is relatively low. A lower effective mass means that the charge carriers can accelerate more easily under an electric field, enhancing their mobility.

Strong Light Absorption:
Perovskites are excellent light absorbers, generating a large number of charge carriers upon illumination. This high carrier generation rate, combined with efficient transport properties, contributes to their high performance in optoelectronic applications.

Soft Lattice:
The relatively soft lattice of perovskites allows for dynamic structural rearrangements that can facilitate charge transport. The material's ability to dynamically adjust its structure can help mitigate the effects of any potential barriers to charge movement. These combined properties make perovskites highly effective for use in a variety of electronic and optoelectronic applications, including high-efficiency solar cells, light-emitting diodes (LEDs), and other devices where efficient charge transport is crucial.

The ability of electrons to move almost freely within perovskite materials significantly impacts their interaction with electromagnetic waves, particularly in the context of optoelectronic applications like solar cells and light-emitting diodes (LEDs). Here are several ways in which this property relates to the interaction with electromagnetic waves:

Efficient Absorption: The electronic band structure of perovskites, with their direct band gaps, allows for efficient absorption of photons (electromagnetic waves) across a broad spectrum. This means that when light strikes the perovskite material, it can be readily absorbed and generate electron-hole pairs (excitons).

Effective Charge Separation: Once the photons are absorbed and excitons are generated, the high mobility of electrons and holes in perovskites ensures that these charge carriers can quickly separate and move towards their respective electrodes. This is crucial for the efficiency of devices like solar cells, where separated charges need to be collected to generate electric current.

Strong Photoluminescence : In LEDs and other light-emitting devices, the free movement of electrons and holes allows them to recombine efficiently to emit light. The high defect tolerance and strong spin-orbit coupling in perovskites further enhance this process, leading to strong photoluminescence.

Reduced Recombination Losses: High electron mobility reduces the likelihood of recombination losses, where electrons and holes recombine without contributing to electric current or light emission. This is important for maintaining high efficiency in both photovoltaic and light-emitting applications.

Dynamic Response to Light: The rapid movement of electrons in perovskites also allows for a dynamic response to changing light conditions. This property is beneficial for applications that require quick adaptation to varying light intensities, such as in adaptive optics or light sensors.

High Absorption Coefficient: The efficient interaction with light also means that perovskite films can be very thin while still absorbing a significant amount of light. This allows for the creation of lightweight, flexible optoelectronic devices.

In summary, the free movement of electrons in perovskites enhances their ability to interact with electromagnetic waves, leading to efficient light absorption, effective charge separation and transport, strong photoluminescence, reduced recombination losses, and a dynamic response to light. These properties make perovskites highly suitable for a range of optoelectronic applications.

There are several types of perovskites, including : 1. Oxide Perovskites: These are the most common and include materials like the original CaTiO3. They are often used in applications such as catalysts, sensors, and superconductors. 2. Halide Perovskites: These have gained significant attention in recent years, especially for their use in solar cells. Methylammonium lead iodide [(CH3NH3)1+(Pb)2+(I)1-3] is a notable example and has been shown to achieve high efficiencies in converting sunlight into electricity. 3. Organic-Inorganic Hybrid Perovskites: These materials replace (CH3NH3)1+ with an organic molecules, offering tunable properties for various applications in optoelectronics.

Figure 12-37b Perovskite Types [view large image]

Type 2 and 3 are designed as A1+B2+X1-3 with X refers to any Group 17 halogen elements. See Figure 12-37b.

In addition, Perovskites can exist in various dimensional forms, each with unique structural, electronic, and optical properties. These different dimensional forms include three-dimensional (3D), two-dimensional (2D), one-dimensional (1D), and zero-dimensional (0D) perovskites.

Here's an overview of the differences between these dimensional forms:

Three-Dimensional (3D) Perovskites
" Structure: The 3D perovskites have a cubic or pseudo-cubic structure, where the A-site cation is surrounded by a framework of corner-sharing BX6 octahedra (where B is a metal cation and X is a halide anion).
" Examples: The most common example is the hybrid organic-inorganic perovskite, methylammonium lead iodide (MAPbI3).
" Properties:
o High Charge Carrier Mobility: Due to the continuous network of octahedra.
o Good Light Absorption: Suitable for photovoltaic applications.
o Stability: Generally less stable under environmental conditions (moisture, heat) compared to lower-dimensional perovskites.
o Application: Widely used in solar cells, LEDs, and other optoelectronic devices.

Two-Dimensional (2D) Perovskites
" Structure: 2D perovskites consist of layers of corner-sharing BX6 octahedra separated by organic cations. They have a formula of (A')2An-1BnX3n+1, where A' is a bulky organic cation and n represents the number of octahedral layers.
" Examples: Ruddlesden-Popper (RP) phase perovskites like (C4H9NH3)2PbI4.
" Properties:
o Enhanced Stability: Improved resistance to moisture and thermal degradation due to the presence of hydrophobic organic layers.
o Quantum Confinement: Reduced dimensionality leads to quantum confinement effects, which can alter optical and electronic properties.
o Application: Used in photodetectors, LEDs, and sometimes as protective layers in 3D perovskite solar cells.

One-Dimensional (1D) Perovskites
" Structure: 1D perovskites have a chain-like structure, where the BX6 octahedra share corners or edges in a linear arrangement, with organic cations filling the spaces between chains.
" Examples: Perovskites like [NH3(CH2)4NH3]PbI4.
" Properties:
o Highly Anisotropic Properties: Charge transport and optical properties are highly directional.
o Strong Quantum Confinement: Enhanced quantum effects due to the reduced dimensionality.
o Application: Potential applications in nanoscale optoelectronic devices and as components in hybrid perovskite structures.

Zero-Dimensional (0D) Perovskites
" Structure: 0D perovskites consist of isolated clusters or polyhedra, such as single BX6 octahedra, surrounded by organic cations.
" Examples: Cs4PbBr6, where the lead bromide octahedra are isolated from each other. " Properties:
o Discrete Energy Levels: Due to the isolation of octahedra, resulting in unique photophysical properties.
o High Stability: The isolated nature often leads to increased environmental stability.
o Application: Used in light-emitting applications and as additives to enhance the properties of other perovskite materials.

Comparison and Applications " Stability: Generally increases from 3D to 0D due to the increased influence of organic cations that can provide protection against moisture and other environmental factors. " Optoelectronic Properties: 3D perovskites are typically better for applications requiring high charge mobility, like solar cells, whereas 2D and lower-dimensional perovskites are better for applications where stability and specific optical properties are crucial, like LEDs and photodetectors. " Quantum Confinement: Becomes more pronounced as the dimensionality decreases, affecting bandgap and emission properties, making lower-dimensional perovskites suitable for tunable light emission applications.

Figure 12-37c Perovskite Dimensions [view large image]

By tailoring the dimensionality of perovskites, researchers can optimize their properties for specific applications, enhancing their performance in a variety of optoelectronic devices.

Applications for the three types of perovskites:

Oxide Perovskites (ABO₃)

1. Catalysis:
o Used as catalysts in oxidation-reduction reactions.
o Applications in the automotive industry for catalytic converters.
o Electrocatalysts for water splitting and fuel cells.
2. Ferroelectric Materials :
o Used in non-volatile memory devices (FRAM).
o Capacitors and piezoelectric sensors.
o Actuators and transducers in various electronic devices.

	3. Magnetic Materials: o Used in spintronic devices. o Magnetic sensors and memory storage devices. 4. Superconductors: o High-temperature superconductors for power cables and magnetic levitation. 5. Thermoelectric Materials : o Used for power generation from waste heat. o Cooling applications in electronic devices. 6. Optoelectronics : o Used in light-emitting diodes (LEDs) and laser diodes.
Figure 12-37d ABO3 Applications	See Figure 12-37d.

Halide Perovskites (ABX₃)

1. Photovoltaics:
o High-efficiency solar cells.
o Tandem solar cells combining different types of perovskites for enhanced efficiency.
2. Light-Emitting Devices:
o LEDs for displays and lighting.
o Lasers for various optical applications.
3. Photodetectors:
o High-sensitivity photodetectors for imaging and sensing applications.
4. X-Ray Detectors:
o Used in medical imaging and security screening.
5. Radiation Detectors:
o Applications in high-energy physics and space missions for radiation detection.

Accoring to ChatGPT, there are certain ways to address the problem :

1. Encapsulation:
o Barrier Layers: Encapsulating perovskite materials with robust barrier layers can prevent lead from leaching out. These barrier layers can be made from materials like polymers, glass, or other inorganic compounds that are impervious to moisture and oxygen, which can trigger lead release.
o Multi-layer Coatings: Using multiple layers of different encapsulating materials can enhance protection and durability.
2. Lead-Free Perovskites:
o Alternative Cations: Research is ongoing to replace lead with less toxic cations such as tin (Sn), germanium (Ge), or bismuth (Bi). Lead-free perovskites like tin halide perovskites (e.g., CH3NH3SnI3) have shown promise, although they still face challenges in terms of stability and performance.
o Mixed Cation Systems: Some approaches involve mixing lead with other cations to reduce overall lead content while maintaining desirable properties.
3. Recycling and Waste Management:
o Recycling Programs: Establishing recycling programs for perovskite solar panels can ensure that lead-containing materials are safely processed and reused rather than being discarded into the environment.
o Safe Disposal: Developing protocols for the safe disposal of lead-containing perovskite materials can minimize environmental contamination. This includes proper waste management practices and regulatory frameworks.
4. Green Synthesis Methods:
o Non-toxic Solvents: Using environmentally friendly solvents in the synthesis of perovskite materials can reduce the overall toxicity of the production process.
o Low-temperature Processing: Developing low-temperature processing methods can minimize the release of toxic substances and reduce the energy consumption associated with manufacturing.
5. Stabilization Strategies:
o Surface Passivation: Applying surface passivation techniques to stabilize perovskite materials can reduce their degradation and the subsequent release of lead.
o Additives: Incorporating additives that can bond with lead and prevent its release into the environment is another approach. These additives can include compounds that form stable complexes with lead ions.
6. Policy and Regulation:
o Regulatory Standards: Implementing strict regulatory standards for the use and disposal of lead-containing perovskites can help mitigate their impact on health and the environment.
o Environmental Monitoring: Regular monitoring of areas where perovskite materials are used or disposed of can help detect and address any lead contamination early.

7. Public Awareness and Education: o Education Campaigns: Educating manufacturers, users, and the general public about the risks associated with lead in perovskites and the importance of proper handling and disposal can enhance safety. o Labeling and Certification: Developing labeling and certification programs for lead-free or low-lead perovskite products can help consumers make informed choices.

Figure 12-37e ABX3 Apps and Lead Poisoning

By implementing these strategies, the potential harmful effects of lead in halide perovskites can be significantly minimized, paving the way for safer and more sustainable use of these materials in optoelectronic applications.

Organic-Inorganic Hybrid Perovskites (ABX₃)
1. Photovoltaics:
o Solar cells with high power conversion efficiency.
o Flexible and lightweight solar panels.
2. Light-Emitting Devices:
o LEDs for low-cost, high-efficiency lighting and displays.
o Tunable color emission for various applications.
3. Photodetectors:
o Sensitive and fast photodetectors for imaging applications.
o Ultraviolet and visible light detection.
4. Lasers:
o Low-threshold lasers for optical communication and sensing.
o Tunable wavelength lasers for various optical applications.
5. Memory Devices:
o Resistive switching memory devices (ReRAM).
o Potential applications in non-volatile memory storage.

Many Lead-Free (LF) perovskites suffer from lower stability compared to lead-based ones, making it challenging to achieve long-term performance. Researchers are enlisting the Organic-Inorganic Hybrid Perovskites to bypass the problem. There are hybrid materials in ongoing researches that combine perovskites with other compounds to improve stability and reduce toxicity. This approach involves integrating materials that can enhance the overall performance while minimizing the environmental impact. Figure 12-37f shows the various perspectives of the hybrid products (all invoke the carbon C element in the "A" component).

Figure 12-37f LF Perovskites

See "Lead-Free Perovskite Single Crystals: A Brief Review".

Examples of Specific Defects and Their Effects in perovskite
" Vacancies:
o Oxygen Vacancies: Can lead to n-type conductivity and enhanced photocatalytic activity.
o A-site and B-site Vacancies: Can influence the ferroelectric and dielectric properties.
" Interstitials:

	o Cation Interstitials: Can increase the electrical conductivity by providing extra charge carriers. o Anion Interstitials: Can create localized states that affect the optical properties. " Substitutional Defects: o Doping with Foreign Elements: Can tailor the bandgap and electronic properties for specific applications like photovoltaics or LEDs.
Figure 12-37g Defeats	Understanding and controlling these defect-induced properties is crucial for optimizing the performance of perovskite materials in a wide range of applications.

The special properties introduced by defeats :

Defects in perovskite materials can introduce a variety of special properties that can be either advantageous or detrimental, depending on the application. Here are some of the notable properties introduced by different types of defects:
1. Electronic Properties
" Trap States: Defects can create electronic trap states within the bandgap, affecting charge carrier dynamics by trapping and releasing electrons or holes. This can be detrimental for photovoltaic applications but beneficial for certain types of sensors.
" Doping Effects: Certain defects, such as interstitials or vacancies, can act as dopants, increasing the material's conductivity. This is particularly useful in tailoring the electronic properties of perovskites for specific applications.
2. Optical Properties
" Enhanced Light Absorption: Defects can introduce localized states within the bandgap, enhancing light absorption in specific wavelength ranges. This can be useful in designing materials for photodetectors and other optoelectronic devices.
" Photoluminescence Tuning: Defects can affect the photoluminescence properties by altering the emission spectrum, which is beneficial for applications like light-emitting diodes (LEDs) and lasers.
3. Structural Properties
" Lattice Distortions: Defects can cause local lattice distortions, which can affect the mechanical properties of the material, such as its hardness and elasticity.
" Phase Stabilization: Certain defects can stabilize or destabilize specific crystallographic phases, impacting the material's structural stability and phase transitions.
4. Chemical Properties
" Reactivity: Defects can increase the chemical reactivity of perovskites, making them more susceptible to reactions with environmental agents like water and oxygen. This can be useful for catalytic applications but detrimental for stability in devices.
" Ion Migration: Defects can facilitate the migration of ions within the lattice, which can be useful for applications like solid-state batteries and ionic conductors.
5. Dielectric and Ferroelectric Properties
" Enhanced Dielectric Properties: Certain defects can enhance the dielectric constant of perovskites, making them suitable for high-k dielectric applications in capacitors and other electronic components.
" Ferroelectricity: Defects can influence the ferroelectric properties by pinning or moving domain walls, affecting polarization switching behavior. This is important for memory devices and sensors.
6. Magnetic Properties
" Magnetism: Defects, such as oxygen vacancies, can induce magnetism in otherwise non-magnetic perovskites, opening up possibilities for spintronic applications.
7. Surface Properties
" Catalytic Activity: Surface defects can enhance the catalytic activity of perovskite materials, making them useful in applications like photocatalysis and electrocatalysis.
" Surface Energy and Wetting: Defects can alter the surface energy and wetting properties, which can influence the material's interaction with other substances, important for coating and adhesion applications.
8. Thermoelectric Properties
" Improved Thermoelectric Performance: Defects can scatter phonons and reduce thermal conductivity while maintaining or enhancing electrical conductivity, improving the thermoelectric performance of perovskites.
9. Environmental Stability
" Degradation Pathways: Defects can either accelerate or decelerate environmental degradation. For example, certain defects can enhance the material's resistance to moisture and oxygen, improving its stability for long-term applications.

Calculating the degree of defects and the tolerance in perovskite materials involves several characterization techniques and theoretical approaches. Here is just an example of using the Goldschmidt Tolerance Factor t to predict the viability of the perovskite structure :

Figure 12-37h Torlerance	Figure 12-37h shows the limitation of "Tolerance" and the rather property of perovskites.

Perovskite materials can exhibit a variety of defects, each affecting their properties in different ways. Here are the main types of defects commonly found in perovskites:

Point Defects
1. Vacancies:
o A-site Vacancy: Missing cation at the A-site of the perovskite structure.
o B-site Vacancy: Missing cation at the B-site.
o Anion Vacancy: Missing anion (typically oxygen or halide) in the lattice.
2. Interstitials:
o A-site Interstitial: Extra cation at an interstitial site.
o B-site Interstitial: Extra cation at an interstitial site.
o Anion Interstitial: Extra anion at an interstitial site.
3. Substitutional Defects:
o A-site Substitution: A different cation replaces the A-site cation.
o B-site Substitution: A different cation replaces the B-site cation.
o Anion Substitution: A different anion replaces the regular anion (e.g., a halide replacing an oxygen atom).
Extended Defects
4. Dislocations:
o Edge Dislocation: A defect where an extra half-plane of atoms is inserted into a crystal.
o Screw Dislocation: A defect where the crystal layers spiral around a line defect.
5. Grain Boundaries:
o The interface between two grains or crystallites with different orientations. These can act as recombination centers for charge carriers.
6. Twin Boundaries:
o A specific type of grain boundary where the lattice structure on either side of the boundary is a mirror image.
Planar Defects
7. Antiphase Boundaries:
o A defect where there is a shift in the phase of the crystal lattice, often resulting in a mismatch in atomic positions.
Complex Defects
8. Defect Clusters:
o Groups of point defects that form a complex, such as a vacancy cluster or interstitial cluster.
9. Schottky Defects:
o A pair of vacancies, one cation and one anion, that maintains charge neutrality.
10. Frenkel Defects:
o A cation or anion vacancy paired with an interstitial of the same species.
Electronic Defects
11. Deep-Level Defects:
o Defects that introduce electronic states deep within the bandgap, acting as recombination centers for charge carriers.
12. Shallow-Level Defects:
o Defects that introduce electronic states near the band edges, often acting as dopants that can enhance conductivity.
Surface Defects 13. Surface Vacancies:
o Missing atoms or ions at the surface of the material.
14. Surface Reconstruction:
o Rearrangement of atoms at the surface to minimize energy, leading to different surface properties compared to the bulk material.
Vacancy Ordered Phases
15. Ordered Vacancies:
o In some perovskites, vacancies can order themselves in a regular pattern, creating a distinct phase with different properties from the disordered phase.
Hybrid Organic-Inorganic Perovskites (Additional Defects)
16. Organic Cation Defects:
o In hybrid perovskites, defects can involve the organic cations (e.g., methylammonium), such as missing or misplaced organic molecules.
17. Hydration Defects:
o Interaction with water can lead to hydration of the material, creating new defects or altering existing ones.
Understanding and controlling these defects is crucial for optimizing the performance of perovskite materials in applications such as solar cells, LEDs, and other electronic and optoelectronic devices.

Defects in perovskite materials can arise from both natural processes and synthetic procedures. When considering perovskites taken directly from the soil, these natural processes are indeed a significant factor. Here are some natural processes that can cause defects in perovskite minerals:

Natural Processes Leading to Defects in Perovskites
1. Geological Formation:
o Pressure and Temperature Variations: During the formation of perovskite minerals in the Earth's crust, varying pressures and temperatures can lead to defects such as dislocations and vacancies.
o Chemical Environment: The presence of different elements and compounds during the formation process can lead to substitutional defects where foreign atoms replace the original perovskite components.
2. Radiation Damage:
o Cosmic and Terrestrial Radiation: Exposure to cosmic rays and natural terrestrial radiation can create point defects such as vacancies and interstitials by displacing atoms from their lattice positions.
3. Mechanical Stress:
o Tectonic Movements: Natural tectonic activity can introduce mechanical stress in the mineral, causing dislocations, grain boundaries, and other extended defects.
4. Weathering and Erosion:
o Chemical Weathering: Interaction with water, oxygen, and other chemical agents can lead to the formation of defects such as vacancies and interstitials, and even alter the chemical composition of the perovskite.
o Physical Weathering: Processes like freeze-thaw cycles and abrasion can introduce mechanical defects such as cracks and dislocations.
5. Impurities:
o Incorporation of Foreign Elements: Naturally occurring impurities present in the soil can be incorporated into the perovskite lattice, leading to substitutional defects and changes in the material's properties.

Defects in Synthetic Perovskites
In addition to naturally occurring defects, synthetic perovskites, which are often used in technological applications, can also have defects introduced during their fabrication. These defects can result from:
1. Synthesis Conditions:
o Temperature and Pressure Control: Imperfect control of temperature and pressure during synthesis can lead to defects similar to those in naturally formed perovskites.
o Chemical Precursors: The purity and stoichiometry of the chemical precursors used can affect the defect concentration and type.
2. Processing Techniques:
o Solvent Quality: The type and quality of solvents used in solution processing can influence defect formation.
o Deposition Methods: Techniques such as spin coating, vapor deposition, and others can introduce defects depending on the uniformity and rate of deposition.
3. Annealing:
o Post-synthesis Treatments: Annealing conditions (temperature, atmosphere) can either heal or create defects in the material.
4. Environmental Exposure:
o Humidity and Oxygen: Exposure to moisture and oxygen during and after synthesis can introduce defects, particularly in hybrid organic-inorganic perovskites.

Mitigation of Defects
Whether in naturally occurring or synthetic perovskites, researchers employ various strategies to mitigate defects:
1. Material Purification: Ensuring high purity of starting materials to minimize impurity-related defects.
2. Optimized Synthesis Conditions: Careful control of synthesis parameters such as temperature, pressure, and atmosphere.
3. Post-Synthesis Treatments: Techniques like annealing and surface passivation to reduce defect concentrations.
4. Protective Coatings: Applying coatings to protect perovskites from environmental degradation.

Understanding the origins of defects and developing strategies to control them are crucial for enhancing the performance of perovskite materials in various applications.

There are several perovskite products available on the market, particularly in the field of solar energy. Perovskite materials have shown great promise in the development of high-efficiency, cost-effective solar cells. Some of such perovskite products include:

* Perovskite Solar Panels: A few companies are in the process of commercializing perovskite solar cells, offering them as either standalone products or as tandem layers on traditional silicon solar cells to enhance efficiency.
* Perovskite Ink and Coatings: These are used in research and development settings and for prototype manufacturing of perovskite solar cells.

	* Perovskite-based LED Lights: Although less common than solar cells, perovskite LEDs (light-emitting diodes) are emerging in the market, offering high efficiency and potentially lower production costs. * Perovskite Materials for Research: Raw materials and pre-fabricated perovskite layers are available for academic and industrial research purposes.
Figure 12-37i Solar Cell Market	The commercial availability of these products may vary by region and the maturity of the technology, as perovskite solar cells are still relatively new compared to traditional silicon-based solar technology.

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